1. Field of the Invention
The present invention relates to a method and apparatus for recording information in which irradiation of an energy beam such as a laser beam or an electron beam causes a change in optical characteristics caused by a change in arrangement of atoms in a recording layer of a recording medium such as an optical disk so as to repetitively record and erase information represented by the lengths of optical marks.
2. Description of the Related Art
Most of recording media (e.g., phase transition type optical disks) which are conventionally and popularly developed and capable of recording/erasing information have a structure shown in FIG. 13. That is, a substrate 1 consists of glass or a plastic material (e.g., a polymethyl methacrylate resin or a polycarbonate resin), and an inorganic protective layer (e.g., a metal or metalloid oxide film, a fluoride film, a sulfide film, a nitride film, or the like) 2, a recording layer 3, an inorganic protective layer 4, and a reflective layer 5 are stacked on one surface of the substrate 1 to form a recording medium 6. Note that the inorganic protective layers 2 and 4 are formed to prevent a change in the recording layer 3 over time. The recording layer 3 is formed by a deposition method such as vacuum deposition or sputtering.
Information can be recorded/erased using the above recording medium 6 as follows. A laser beam is radiated on the entire surface of the recording medium 6 to heat the recording medium 6 and to set the recording layer 3 with high crystallinity (state in which atoms are relatively regularly arranged, and this state will be referred to as a crystalline state hereinafter). In order to record information, a short, strong pulse beam is radiated on the recording medium 6 to heat and abruptly cool the recording layer 3. At this time, a portion irradiated with the pulse beam is set to low crystallinity (state in which atoms are irregularly arranged, this state is to be referred to as an amorphous state hereinafter).
In both the crystalline and amorphous states, the structures of the arrangements of atoms are different from each other. For this reason, optical characteristics (transmittance and reflectance) are changed, so that information can be recorded. In this manner, the recorded information can be erased such that a long, weak pulse beam is radiated on the recording portion to heat and gradually cool the recording portion, because the state of the recording portion is returned to the crystalline state, i.e., an original state.
The above state can be realized by a so-called overwrite operation. According to the overwrite operation, a laser beam obtained by superposing a short, strong pulse beam of a recording power level on a weak continuous light beam of an erasure power level as shown in FIG. 14 is used, and a new recording portion is formed while an old recording portion (amorphous state) is erased (set in a crystalline state).
In order to record information represented by the lengths of optical marks in a recording medium using a pulse beam having the rectangular waveform shown in FIG. 14, the recording layer of a portion irradiated with the laser beam generates heat and is melted. In this case, since the laser beam has a high power, an amount of heat generated by the recording layer is larger than the amount of latent heat required to melt the recording layer and the flow rate of heat to the reflecting and protective layers, and heat is accumulated in the recording layer. In completion of irradiation of the recording pulse beam, a melted region extends in a direction perpendicular to the recording tracks of the phase transition type optical disk (recording medium), and the shape of each recording mark becomes a so-called water-drop shape having a thin front end portion and a thick rear end portion. When the mark having the water-drop shape is reproduced as described above, the leading and trailing edges of the waveform of the reproduced signal have different shapes, and the reproduced signal is distorted. For this reason, jitters caused by a slight difference between the phases of a recording signal and the reproduced signal are increased, and recording and reproduction errors are increased, thereby causing a decrease in margin.